Setting USE_EXPAND flags in package.use

This has apparently been supported in Portage for some time, but I only learned it recently from a gentoo-dev mail: you do not have to write down the expanded USE-flags in package.use anymore (or set them in make.conf)!

For example, if I wanted to set some APACHE2_MODULES and a custom APACHE2_MPM, the standard package.use entry would be something like: